The particular option you choose to get new players depends on the current needs of your team.

If you're in need of some major new talent, you may opt to bid on high performing players.

If you notice there is a lack of offense for one of your positions and another team, you might opt to propose a trade.

You have a minor league team (Triple-A) that you can swap players between as well. Calling up minors leaguers will require sending down a player to maintain your team roster at 24 players.

When active, there is also the option of drafting new talent from college. By default you can draft two new players per season.
Click one of the links above for more detail on why / how to utilize each of these methods.